Patients who have undergone surgery or who have certain respiratory diseases or paralysis often have problems breathing properly and therefore must be provided with artificial respiration. The respirator used must be capable of being adjusted to provide the required amount of oxygen at an optimum frequency. The volume and frequency required not only varies with different patients, but can also vary within a single patient as the condition of the patient varies. In prior art respirators, changes must be made manually. This requires almost constant attention to the patient by a doctor or nurse, and the amount of change, when required, is effected by trial and error.
Attempts to provide automatic control of a respirator based on the condition of the patient have been described. One prior art system that has been described uses the concentration of carbon dioxide in the exhaust of the patient to control the operation of the respirator by switching the respirator on or off. See U.S. Pat. No. 4,537,190 to L. Caillot et al., issued Aug. 27, 1985, entitled "Process and Device for Controlling Artificial Respiration". See also, Y. Mitamura et al., "A dual control system for assisting respiration", MEDICAL & BIOLOGICAL ENGINEERING, Vol. 13, No. 6, pages 846-854. Other systems which have been described use both the concentration of carbon dioxide and oxygen in the exhaust, but these are used to control outputs of gas mixers, not outputs of an artificial respirator. See, e.g., M. H. Giard et al., "An Algorithm for Automatic Control of O.sub.2 and CO.sub.2 in Artificial Ventilation", IEEE TRANSACTION ON BIOMEDICAL ENGINEERING, Vol. BME-32, No. 9, September 1985, pages 658-667, and C. Yu et al., "Improvement in Arterial Oxygen Control Using Multiple-Model Adaptive Control Procedures", IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Vol. BME-34, No. 8, August 1987, pages 567-574. However, all of these systems have problems in that they either do not take into consideration all of the relevant physical conditions of the patient or the techniques employed are directed toward controlling the output of the breathing gas supply rather than the artificial respirator.